One characteristic variable of lasers is radiant power. In case of high-power continuous-wave lasers, radiant power can be in the kilowatt range and thereabove; in case of pulsed lasers, peak power is even frequently in a range far above 1 megawatt. On the one hand, these high radiant powers open up many industrial applications for lasers; on the other hand, on account of these high outputs, there is the danger of destruction of optical components. Therefore, great care is required in the selection of optical parts for lasers of high radiant power.
The beam product (.theta..multidot.D) is another characteristic variable. For a beam which is rotationally symmetrical with respect to the axis, this is the product of the diameter D in meters with the full angular divergence .theta. in radians of the beam at the laser output. In the more general case of beams that are not rotationally symmetrical, the chord of the beam and/or the angular divergence of the radiation depending on the angle .phi. perpendicular to the beam axis, .theta..multidot.D denotes the maximum of .theta.(.phi.).multidot.D(.phi.) in dependence on .phi., wherein .theta.(.phi.) means the full angular divergence in rad of the radiation in dependence on .phi., and D(.phi.) means the chord passing through the apex of angle .phi., in m, depending on .phi., and .theta.(.phi.) and D(.phi.) are referred to the output of the laser. In this definition, the beam product, though based on the laser output, need only rarely be measured directly at the laser output to determine this product. This is so because the beam product remains preserved if guidance of the beam is carefully executed. In other words, the beam product, with observance of known rules, is a preserved variable characterizing the radiation and can be measured even at a large distance from the laser. Especially in case of power lasers, the beam product can be readily determined because the radiation intensity of these lasers drops abruptly in most cases as soon as a critical value for the distance from the beam axis has been exceeded, i.e. the contour of the laser beam in the plane perpendicular to the beam axis is sharply defined. However, even if the intensity does not drop abruptly with increasing distance from the beam axis, the beam product can be determined with adequte accuracy--for example by defining the contour of the laser beam by the 1/e.sup.2 value of the radiation intensity, based on the intensity maximum.
For the radiation at the output of a laser of high power, values of 1.multidot.10.sup.-3 m to 5.multidot.10.sup.-2 m are usual for the beam diameter, and values of 1.multidot.10.sup.-3 rad to 5.multidot.10.sup.-2 rad are customary for the full angular divergence; in special cases, the values can even lie far above these numbers. Values typical for the beam product of high-power lasers are 2.multidot.10.sup.-5 rad.multidot.m to 3.multidot.10.sup.-4 rad.multidot.m; for example, for Nd lasers in continuous-wave operation with outputs of 100 W up to 1 kW, values of 5.multidot.10.sup.-5 rad.multidot.m to 2.multidot.10.sup.-4 rad.multidot.m are usual, and for Nd lasers in continuous-wave operation with more than 1 kW, values of up to 3.multidot.10.sup.-4 rad.multidot.m are customary.
A low beam product is equivalent to good quality of laser radiation. For usage aspects, a good beam quality, or a low beam product, is of essential importance--many uses are only opened up if the beam product is adequately low (for example: H. Weber, "High Power Nd-Lasers for Industrial Applications", Proc. SPIE 650, p. 92, 1986). For this reason, considerable expenses are incurred in the development of lasers for medical and industrial applications in order to reduce the beam product of laser radiation.
With adequate care, radiation can be guided by means of optical components (lenses, mirrors, etc.) without substantially altering its beam product. If such care is lacking, the beam product is enlarged. A reduction of the beam product, in contrast thereto, is possible in principle only with toleration of, in most cases, considerable radiation losses. The manufacturers and users of lasers thus do not accept beam guidance systems which impair the quality of the laser radiation. Thus, if a flexible fiber is to be utilized for transmitting radiation, a decisive advantage resides in the feature that the fiber will transmit the radiation in such a way that the beam product at the emergence end of the fibers, as compared with the value at the inlet end, is only slightly increased.
In optical fibers, the minimum bending radius R.sub.min is another decisive characteristic variable. This is the smallest radius to which a fiber can be bent without being destroyed by bending-induced mechanical stresses and without bending-induced radiation losses destroying the fiber or its surrounding, or endangering the fiber or its surrounding, or too greatly reducing the transmission efficiency of the fiber.
The optical fibers presently utilized for power transmission have an elastic behavior upon bending. High mechanical stresses occur, especially on the fiber surface (cladding surface) by bending the fiber to small radii, whereby the fiber can break. However, with the prevailing manufacturing processes for optical fibers, particularly for glass fibers, very high material strengths are achieved. Unless the original strength of the fiber fades, for example due to aging the danger of breakage has ultimately only a subordinate significance as compared with bending-induced radiation losses. In any event, the fiber system must, however, contain safety components interrupting the supply of radiation in case of a fiber break. Suitable detectors for leakage radiation and fiber breakage are conventional.
Bending-induced radiation losses can be "actual bending losses" or so-called transition losses. The radiation loss is generally converted into heat in general still within the fiber or in its immediate surroundings, i.e. the aspects of reduced transmission efficiency of the fiber (that is the ratio of the radiant power transmitted by the fiber to the coupled-in power) and of thermal stressability of the fiber must be taken into account.
Actual bending losses arise because the radiation in the core of bent fibers couples to so-called leakage waves. These losses, however, do not occur along a short fiber section--rather, it is characteristic that they are distributed essentially along the entire bent fiber section. Therefore, by suitable cooling of the fiber, its thermal load produced by bending losses can generally be reduced to such an extent that there is no danger to the fiber. The limits of technical usability of the fiber here arise on account of the diminished transmission efficiency of the fiber or due to the fact that cooling of the fiber becomes too expensive for practical purposes.
Transition losses are produced if the bending of the fiber changes (alteration of bending radius, etc.). The characteristic property of these losses is that they occur along short fiber sections directly at or downstream of the transition point. For this reason, in case of high radiant outputs, the local thermal stress on a fiber can be so high that the fiber cannot be cooled with an expenditure tolerable for practical utilization. This can also be true in case the reduction in transmission efficiency due to transition losses is negligible for the practical applications. In general, at high radiation outputs, the limitations imposed by the transition losses are more significant than those imposed by the bending losses.
It is known experimentally and theoretically that the bending and transition losses depend greatly on the type of fiber and on the bending radius. In most cases, a permissible minimum bending radius can be indicated for a fiber so that it is ensured that these losses are sufficiently small as long as the bending radius of the fiber is larger than this minimum value (in the following: minimum bending radius of the fiber). Also, devices are known by means of which a fiber can be bent only to radii larger than a predetermined minimum value.
The required minimum bending radii of the fibers are determined by the usage situations. It has turned out, for fibers used to transmit high-output radiation, that minimum bending radii of .apprxeq.0.3 m are sufficient for connecting stationary appliances; that, for connecting mobile devices, .apprxeq.0.15 m to 0.3 m is enough; for inscription purposes and for industrial robots, 0.1 m to 0.15 m, and that minimum bending radii of .apprxeq.0.05 m will be adequate for extracorporal medical uses of power lasers. Although bending radii of less than 0.05 m are required for invasive medical applications, it is not clear at this time whether power lasers will at all be employed for such purpose.
If it were possible to use a fiber with a bending radius smaller than that needed for the particular usage, no advantage would be gained. Instead, in most cases express drawbacks would even have to be suffered with respect to the design of the fiber and the radiation distribution in the fiber. Therefore, it is advantageous to manufacture fibers which can be bent maximally exactly at the radius needed by the usage situation--of course considering adequate safety--and to ensure, optionally by mechanical devices, that this bending radius when using the fibers will not be at a lower value. An industrially usable fiber, therefore, should exhibit the following properties:
a high optical destruction threshold and/or a high maximally transmissible radiant power, PA1 no impairment of the beam quality by the transmission, PA1 high transmission efficiency, and PA1 a maximally accurate attainment and/or maintenance of the minimum bending radius of the fibers required for the practical applications. PA1 (a) the numerical aperture determined by n.sub.2 and n.sub.1, EQU NA.sub.1 =(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2 PA1 amounts to 1.5 to 2.9 times the cube root of (.theta..multidot.D)/R.sub.min, but is larger than 0.05 and smaller than 0.28, wherein (.theta..multidot.D) is the product, known as beam product, of the full angular divergence .theta. in rad with the diameter D in m of the radiation to be coupled into the fiber, these data being based on the output of the radiation source, and R.sub.min in m is the minimum bending radius of the fiber required by the respective usages, PA1 (b) the diameter of the core D.sub.1 in m is 0.45 to 0.65 times the quotient of (.theta..multidot.D)/NA.sub.1, PA1 (c) the numerical aperture determined by n.sub.3 and n.sub.2, EQU NA.sub.2 =(n.sub.2.sup.2 -n.sub.3.sup.2).sup.1/2 PA1 amounts to the single to double value of NA.sub.1, PA1 (d) and that the outer diameter D.sub.2 in m of the radiation-conducting intermediate layer is smaller than 0.15.multidot.R.sub.min .multidot.NA.sub.2.sup.2.
Fibers and, respectively, fiber systems for transmission of laser radiation of high radiant power have been known in the patent and other literature.
U.S. Pat. No. 4,707,073 describes a fiber-optic beam guidance system for transmitting high radiation outputs wherein the fiber is cooled to prevent its overheating on account of radial coupling-out losses. The optical fiber has a very large core diameter of 1.multidot.10.sup.-3 m and a cladding thickness of 1.multidot.10.sup.-4 m (fiber SM 1000 of Dainichi-Nippon Cables, Tokyo, Japan). Thereby, the fiber surface becomes large, and heat transfer between fiber and cooling medium is satisfactory--however, the large diameter of this fiber prevents bending at the radius needed for many applications. On account of the large core diameter and the large numerical aperture, the fiber furthermore has the drawback that it cannot transmit laser radiation with the beam quality necessary for the applications.
With intensive cooling of the fiber, the problem can also occur that the mostly liquid coolant (e.g. water) penetrates the fiber coating, damages the fiber surface (e.g. by penetration of OH ions by diffusion, or by leaching), and reduces the mechanical strength of the fiber. Attempts have been made to counteract such drawbacks by using denser coating materials, such as metals, for example. European Patent Application EP No. 00 63 581 describes, for example, an optical fiber having even two metallic coatings in order to further enhance the density of the fiber shielding. With the use of such fibers for power transmission, the diameter of the fibers could, under certain circumstances, be markedly reduced. However, the realizable metal coatings do not exhibit the desired properties. Besides, other problems are caused by the metallic coatings: in order to attain appreciable layer thicknesses (about 50.multidot.10.sup.-6 m), the metals must be applied in the liquid phase--however, due to thus-occurring thermal and mechanical stresses, the fiber properties are adversely affected. The properties (e.g. thermal expansion coefficient and modulus of elasticity) of the metals and fiber materials (mostly glasses) in question are greatly different. Thereby, mechanical stresses arise in the fiber which stresses, inter alia, evoke high microbend losses (for example: T. Shiota, H. Hidaka, 0. Fukuda and K. Inada, "High Temperature Effects of Aluminium Coated Fibers", IEEE J. Lightwave Techn. 4 : 1151, 1986). Last but not least, the radiation leaving the core and reaching the fiber coating is absorbed by the metal coating along a very short length--normally, the use of a metal coating has the result that the radiation losses are converted into heat with more intense localization; the thermal stress on the fiber is even increased.
Under the designations of "double-clad fibers" and "multiple-clad fibers", the literature discloses fibers having additional layers between core and cladding. However, such fibers are predominantly monomode fibers which are not suitable for transmission of high radiant outputs on account of the small core diameter. The purpose of the additional layers is the improvement of dispersion or the influence to be exerted on the cut-off condition of fibers for communications technology, these fibers transmitting only low-power radiation (several milliwatts).
In most cases, "depressed-cladding" fibers are involved (for example: H. R. D. Sunak and S. P. Bastien, "Universal Single-Mode Dispersion-Flattened Fluoride Fibre Designed for Optimum Performance from 1.5 to 2.9 .mu.m", Electron. Lett. 24 : 879, 1988). In these fibers, the index of refraction of the additional intermediate layer is lower than the index of refraction of the innermost cladding layer. Similar considerations apply also to specific designs of multimode fibers: U.S. Pat. No. 4,691,990, for example, describes a fiber wherein, for producing a high numerical aperture, an additional layer of a glass having an especially low index of refraction is applied to the core; however, this layer is thin and is surrounded by another rugged glass jacket of higher index of refraction, for example because the mechanical properties of the material of this layer are unfavorable. Fibers wherein the index of refraction of the intermediate layer is lower than the index of refraction of the surrounding cladding exhibit, however, the drawback that the intermediate layer cannot conduct any radiation. Apart from this, fibers wherein, as is customary in industry, a maximally high numerical aperture is desired, cannot transmit radiation with the beam quality required for the applications, precisely because of the high numerical aperture.
Double- and multiple-clad fibers are also known wherein the refractive indexes of layers diminish radially from the inside toward the outside. However, in these fibers, the difference of refractive indexes between the material of the core and that of the first layer surrounding the core is larger than the refractive index differences between the materials of the subsequent layers (for example: L. Cohen, D. Marcuse and W. L. Mammel, "Radiating Leaky-Mode Losses in Single-Mode Lightguides with Depressed-Index Claddings", IEEE Journal of Quantum Electronics QE-18, p. 1467, 1982). It is furthermore characteristic that the index of refraction of the layers, after having reached a minimum, increases again radially, as described in U.S. Pat. No. 4,691,991 for a monomode fiber (such fibers, on account of the typical shape of the radial refractive index profile, are also called "W fibers").
The conventional double- and multiple-clad fibers have several advantageous properties for transmitting monomode radiation, but they are useless for transmitting multimode radiation, for example because the bending losses of the higher modes are high (for example: H. Schwierz and E.-G. Neumann, "Bend Losses of Higher-Order Modes in Dispersion Flattened Multiple Clad Optical Fibres", Electron. Lett. 24 : 1296, 1987). Besides, in multimode fibers, with the refractive index ratios of the double- and multiple-clad fibers, the radiation entering into the layers between core and cladding would very quickly exit from the fiber. Therefore, these layers could not be exploited in fibers for the transmission of high radiant outputs.
European Patent Application EP No. 02 34 233 describes a fiber for transmitting high radiant outputs wherein, in order to reduce radial radiation losses, the cladding consists of a radial layered system which acts in a similar way as the layered system of a dielectric mirror. For this purpsoe, it is necessary for the thickness of the layers to have the size of the radiation wavelength (i.e. less than 10.sup.-5 m), and for the differences in refractive index between neighboring layers to be large. The substantial disadvantage of this fiber resides in that the required tolerances are difficult to maintain from a manufacturing viewpoint, and that the fiber cannot be attuned to the beam product of the radiation and therefore cannot transmit radiation with the beam quality required for the usage situations.